The present invention relates generally to laser radar systems for measuring winds and, more particularly, to an apparatus and method for measuring the direction of the atmospheric flow field.
Laser radar systems are used for ground wind monitoring in order to improve the accuracy of free rockets and artillery. These ballistic devices are susceptible to deflection due to crosswinds, particularly during the early, low-velocity portions of their trajectories, and a knowledge of the wind velocity within, for example, one hundred meters of their launch sites, permits the initial aiming direction to be compensated for such wind. Ground wind measurements are also becoming increasingly required at airports.
In an illustrative laser radar system used to measure wind velocity, a laser beam is scanned such that its focus describes a closed path which is generally parallel to the earth's surface. Reflections from aerosols at the scanned focal point exhibit a Doppler shift in frequency according to the velocity of the aerosols; this velocity can be measured by beating the return signal with the transmitted beam.
Conventional CO.sub.2 laser systems designed to measure winds do not give the direction of the atmospheric flow field. There is an ambiguity in algebraic sign, since these systems do not provide sufficient information on the phase of the return signal and the range from which it was obtained. These systems have a range resolution longer than the wavelength of the radiation, and the ambiguity cannot be resolved by going to smaller range resolutions. Even if this were the case, relative motion of particles within the esolution volume would prevent continuous monitoring of the phase.
The usual way to resolve the ambiguity is to introduce a frequency translation either by means of a second laser offset with respect to the first, or by the addition of an acousto-optic frequency translator such as a Bragg cell or a Raman-Nath cell. Both of these techniques provide fairly accurate determination of the frequency, and the velocity ambiguity can be resolved down to a fraction of a foot per second. Moreover, Bragg cells permit continuous measurement of the wind flow direction. However, the price of these two techniques is quite high. They also suffer from a rather strong feedthrough problem, and do not work near the frequency translation.